As an example of such lighting device for inspection for use in e.g. inspection of an appearance of a product, it is possible to cite a coaxial lighting device shown in Patent Document 1, in which an imaging direction is in agreement with a lighting direction for an inspection object. This coaxial lighting device includes a light source configured to emit inspection light in a direction parallel with an inspection object face of the inspection object, and a half mirror disposed with a tilt between the inspection object and an imaging (image pick-up) device disposed upwardly of the inspection object and configured to reflect the inspection light toward the inspection object and to transmit the reflected light from the inspection object toward the side of the imaging device.
Incidentally, in recent years, there is a demand for ability of detecting a feature point such as a defect difficult to detect with using the above-described inspection lighting device through a captured image thereof. More particularly, since the surface property of a product as the inspection object is not a perfect mirror surface, precision control of e.g. an optical axis or shape of an irradiation solid angle for obtaining desired gradation information on a feature point on the inspection object surface is difficult. Thus, even if the inspection light can be irradiated, as there occurs significant contrast variation depending on at what position on the inspection object the feature point is present, identification of the feature point is difficult.
For instance, it is conceivable to increase the inspection precision by limiting the irradiation area of the inspection light to the inspection object only with use of an aperture stop or the like, thereby to decrease stray light which is a reflected light or scattered light from an non-inspection target object.
However, even when reduction of such stray light coming into the imaging device is made possible, in case of a very small defect etc., there occurs significant variation in the brightness of captured image, which makes detection thereof as a defect impossible.
More specifically, even when there occurs a small change in the reflection direction of irradiated inspection light due to presence of e.g. a small defect on the inspection object, if this change is in such a range as confined within an observation solid angle of the imaging device, the brightness of the captured image will remain unchanged, regardless of presence/absence of the defect. Or, if the irradiation solid angle of the inspection light is large and the tilt of its optical axis differs among respective points on the inspection object, a small change in the reflection direction can not be grasped as a change of light amount within the observation solid angle of the imaging device; and moreover, the light amounts within the observation solid angle of the imaging device will vary irregularly for/among the respective points. Consequently, the machine vision is unable to accurately detect such minor defect or the like in the inspection target range.